This project has two long-term objectives, both of which are related to an unexpected observation made recently in my laboratory. That observation is that, under appropriate conditions, inputs over unmyelinated (C) peripheral afferent fibers have inhibit the nociceptive flexion withdrawal reflex (FR). To date we have shown that the FR recorded from semitendinosus and evoked by stimulating the tibial nerve is inhibited by C fibers in the spinal, but not decerebrate, state. Conversely, the FR recorded from tibialis anticus and evoked by stimulating the tibial nerve can be inhibited by C fibers in the decerebrate, but not spinal, state. Both observations were completely unexpected; there is nothing in the literature to suggest that C fibers should inhibit the FR under any circumstances. The first objective of the project is to look for inhibitory effects of peripheral C fibers on the FT using a variety of nerve/muscle pairs. A thorough understanding of when such inhibition would and would not be expected is critical for the second objective, which is to begin to search for singly neurons in the spinal cord which may contribute to this inhibition. The studies proposed are important for two reasons. First, they will help us better understand the organization of segmental spinal reflexes, which are considered to be among the simplest of motor behaviors. Second, they will help us to better interpret the FR as an animal model of nociception in humans. The findings described above are just the opposite of what would have been predicted based on our assumption that excitability of spinal interneurons which contribute to the FR parallels that of spinal interneurons which contribute to pain perception. This would suggest that the FR is not as accurate of a model of nociception and is commonly considered.